Prior treatment plants are based on batchwise feeding of the material into a treatment chamber into which chemicals are added. The chemical additives cause reactions in the material resulting in heat generation and evaporation of liquid regulation of the pH value and increase of nitrogen in the final product. Then heat is added to the thus chemically treated mass to evaporate liquid and to achieve a desired percentage of solid matter in the final product.
This prior technique suffers from several drawbacks and deficiencies. Thus, batchwise treatment is not an effective approach. Also, the relatively high temperatures as used (about 425.degree. C.) result in destruction of humic acid and other substances important to soil improvement.
Furthermore, when the reaction processes take place within the mass, gasification of liquid is hampered and the results of the reaction processes generally lowered. Using external heat for gasification/drying and granulation after reaction treatment the temperature should be sufficiently low to avoid loss of the material substances that are valuable to soil improvement. Various low temperature technology processes may be adapted to this process. However, the properties of the material emanating from the reaction treatment would be of great importance, also for the result of this process.
The prior art also includes continuous processes for treating liquid-containing materials such as sewage sludge, as disclosed in U.S. Pat. No. 4,038,180 and EP-A-0 356 781. However, such prior processes suffer from the same drawbacks as discussed above in connection with batchwise material feeding, and in addition they tend to be less efficient owing to the slow throughput needed to ensure a sufficient sludge dwell time for the required reactions and drying to take place.
The main object of the invention is to provide an approach that permits a continuous process, and that does not require external heating of the sludge to effect the necessary evaporation, while keeping the temperature sufficiently low to avoid loss of any of the substances in the material that are valuable for improving soil.
According to the invention this object is achieved through a method and an apparatus as defined in the accompanying patent claims.